1. Field of the Invention
The present invention relates to a process for producing polycarbonate and more particularly to a process for efficiently producing polycarbonate excellent in tone and heat resistance.
2. Description of Related Arts
In general, polycarbonate is produced by reacting a polyhydric phenol, such as dihydric phenol and more hydric phenol, with phosgene in the presence of an alkali. Particularly on a commercial scale, polycarbonate has been efficiently produced by continuous stirred tank-reactor process.
This process has a disadvantage in that polycarbonate having a number of hydroxyl groups as terminal groups is liable to contain an impurity because of its great affinity to water during the washing step, thereby causing problems such as poor tone and poor heat resistance of the polycarbonate product. Moreover, the molecular weight of polycarbonate is not uniform. Polycarbonate containing a relatively large amount of low molecular weight products is liable to stick to the mold at the time of molding, which is responsible for poor tone, and polycarbonate having a wide distribution of molecular weight is responsible for poor heat resistance. If the alkali concentration at the initial stage of the reaction is increased in order to shorten the reaction time, the chloroformate group of polycarbonate oligomer is markedly decomposed. Moreover, the above process has serious problems in the commercial practice thereof that the resistance time is long and a long time is needed in changing the grade of polycarbonate to be produced.
An improved method to overcome the above problems is disclosed in Japanese Patent Publication No. 36554/1977. In this method, an attempt to narrow the molecular weight and to depress the decomposition reaction is made by using a tubular reactor. This improved method, however, is still unsufficiently satisfactory although the object is attained to a certain extent.